System for the transfer and gravitational drainage of a gas in liquid form

Abstract

A transfer system, for transferring liquid-form gas between two liquid-form gas units, includes: a main pipe configured to transfer the liquid-form gas from a source tank of a liquid-form gas source unit to a receiving tank of a liquid-form gas receiving unit, the main pipe including at least a first portion and a flexible second portion; and at least one return pipe configured to convey the liquid-form gas present in the main pipe toward the source tank, the transfer system being configured to drain the liquid-form gas under gravity.

Claims

1. A transfer system for transferring liquid-form gas between two liquid-form gas units, comprising a main pipe configured to transfer the liquid-form gas from a source tank of a liquid-form gas source unit to a receiving tank of a liquid-form gas receiving unit, said main pipe comprising at least a first portion and a flexible second portion, the transfer system comprising an articulated support device for supporting the main pipe, the first portion being secured to the articulated support device and configured to withdraw the liquid-form gas contained in the source tank, wherein the transfer system comprises at least one return pipe configured to convey the liquid-form gas present in the main pipe toward the source tank, the transfer system for transferring liquid-form gas being configured to drain the liquid-form gas present in the main pipe under gravity toward the source tank via the return pipe.

2. The transfer system as claimed in claim 1, wherein the draining of the liquid-form gas is activated by at least one flow controller situated on the return pipe.

3. The transfer system as claimed in claim 1, wherein the return pipe comprises an emergency disconnection device.

4. The transfer system as claimed in claim 1, wherein the return pipe comprises at least a pressure sensor and at least a temperature sensor sensing the pressure and the temperature of the return pipe.

5. The transfer system as claimed in claim 1, wherein the return pipe has a bore section of between 300 mm.sup.2 and 2000 mm.sup.2.

6. The transfer system as claimed in claim 1, wherein the circulation of the liquid-form gas within the main pipe is activated by at least a first valve positioned on the main pipe.

7. The transfer system as claimed in claim 1, comprising a pressurizing line connected to the main pipe and configured to remove the liquid-form gas present in the main pipe and in the return pipe.

8. The transfer system as claimed in claim 1, wherein the return pipe comprises a first end connected to the main pipe and a second end configured to open into the source tank of the liquid-form gas source unit, the second end of the return pipe being vertically lower down than the first end of the return pipe.

9. The transfer system as claimed in claim 8, wherein the first end of the return pipe is connected to the main pipe via a first connection/disconnection device.

10. The transfer system as claimed in claim 1, wherein the flexible second portion is configured to be connected to the header of the third portion by a second connection/disconnection device.

11. The transfer system as claimed in claim 10, wherein the return pipe is connected to the main pipe in the flexible second portion thereof upstream of the header.

12. The transfer system as claimed in claim 10, wherein the return pipe is connected to the main pipe in the third portion thereof downstream of the header of the third portion and upstream of the second valve of the third portion.

13. The transfer system as claimed in claim 1, wherein the flexible second portion of the main pipe comprises a first end and a second end, the first end of the flexible second portion being secured to the articulated support device.

14. The transfer system as claimed in claim 3, wherein the main pipe comprises a third portion, said third portion comprising a first termination provided with a header, and a second termination configured to open into the receiving tank.

15. The transfer system as claimed in claim 14, wherein the third portion comprises at least a second valve positioned between the header of the third portion and the second termination of the third portion.

16. A drainage method for draining a liquid-form gas, implemented by a transfer system for transferring liquid-form gas as claimed in claim 8, and comprising: a first step in which at least the second valve of the third portion of the main pipe is closed, a second step in which the articulated support device supporting the main pipe is raised so as to position the first end of the flexible second portion vertically higher up than the second end of the flexible second portion, a third step in which at least the flow controller of the return pipe is opened.

17. The drainage method as claimed in claim 16, comprising an additional step that comes later than the first step, and during the course of which a gas valve is opened so as to connect a headspace of the source tank to the first portion of the main pipe.

18. The drainage method as claimed in claim 16, comprising a fourth step, consecutive with the third step of heating the main pipe.

19. The drainage method as claimed in claim 16, comprising an additional step that comes later than the first step, and during which at least the first valve of the main pipe is closed.

20. The drainage method as claimed in claim 19, during the course of which a fluid different than the liquid-form gas is injected so as to drive out the liquid-form gas present in the main pipe.

Description

(1) Further features and advantages of the invention will become more apparent through, on the one hand, the following description and, on the other hand, a number of exemplary embodiments given by way of nonlimiting indication with reference to the attached schematic drawings in which:

(2) FIG. 1 is a schematic depiction of a first embodiment of a liquid-form gas transfer system according to the invention, during a liquid-form gas transfer operation,

(3) FIG. 2 is a schematic depiction of the first embodiment of the transfer system during a liquid-form gas drainage operation,

(4) FIG. 3 is a schematic depiction of a second embodiment of the transfer system during the liquid-form gas transfer operation,

(5) FIG. 4 is a schematic depiction of the second embodiment of the transfer system during the liquid-form gas drainage operation,

(6) FIG. 5 is a flow chart of a liquid-form gas drainage method according to the invention,

(7) FIG. 6 is a schematic depiction, with cutaway, of a tank of a transport ship and of a maritime terminal used for loading this tank.

(8) During the course of this description, the terms upstream and downstream will refer to a positioning of elements relative to a direction of circulation of liquid-form gas within the pipes mentioned.

(9) FIG. 1 depicts a first embodiment of a liquid-form gas transfer system 1.

(10) The transfer system 1 transfers a liquid-form gas 31 from a liquid-form gas source unit 2 to a liquid-form gas receiving unit 3. The liquid-form gas source unit 2 comprises a source tank 4 and the liquid-form gas receiving unit 3 comprises a receiving tank 5. It will thus be appreciated that the transfer system 1 collects the liquid-form gas 31 contained in the source tank 4 with a view to conveying it to the receiving tank 5 in order to fill the latter. By way of example, the liquid-form gas source unit 2 and the liquid-form gas receiving unit 3 associated with it may correspond to various examples as set out in the following table:

(11) TABLE-US-00002 TABLE 2 Source unit Receiving unit On-shore reservoir, floating Methane tanker, barge, ship running reliquefaction unit, other on liquid-form gas of the cruise reliquefaction unit, gravity liner, container ship, ferry type platform Methane tanker Barge, ship running on liquid-form gas of the cruise liner, container ship, ferry type Barge Ship running on liquid-form gas of the cruise liner, container ship, ferry type

(12) In FIG. 1, the liquid-form gas source unit 2 may for example be a loading barge or a quayside for the loading of liquid-form gas. The liquid-form gas receiving unit 3 illustrated in FIG. 1 corresponds to a ship that transports liquid-form gas, for example a methane tanker.

(13) In order to transfer the liquid-form gas 31 from the source tank 4 to the receiving tank 5, the transfer system 1 comprises a main pipe 6 that participates in an operation of transferring the liquid-form gas 31. The transfer system 1 also comprises a return pipe 18 that at least partially contributes to an operation of draining the liquid-form gas 31 once the transfer operation is complete.

(14) The main pipe 6 extends overall from the source tank 4 where the main pipe 6 collects the liquid-form gas, to the point at which it opens into the receiving tank 5. In this capacity, the main pipe 6 comprises a first portion 7, a flexible second portion 8, and a third portion 9. The first portion 7 is partially immersed in the liquid-form gas 31 of the source tank 4. The liquid-form gas 31 of the source tank 4 can thus circulate in the main pipe 6 by passing through the first portion 7. The first portion 7 comprises a pump 32 the purpose of which is to draw the liquid-form gas 31 from the source tank 4 into the main pipe 6. The transfer operation is therefore initiated by bringing the pump 32 of the first portion 7 into operation. The first portion 7 outside of the source tank 4 is supported by an articulated support device 26. The articulated support device 26 may for example be a crane positioned at the level of the liquid-form gas source unit 2. The articulated support device 26 comprises a mast 27, a jib 29 and a pivot 28 connecting the jib 29 to the mast 27. The pivot 28 is thus able to cause the jib 29 to pivot with respect to the mast 27. The mast 27 extends mainly vertically and the first portion 7 extends along the mast 27, being for example fixed thereto by any fixing means. The first portion 7 also extends along the jib 29 where it is supported thereby, for example via at least one bearer 30. Thus, when the jib 29 is articulated via the pivot 28, the first portion 7 is driven by the jib 29 via the bearer 30. The first portion 7 also comprises a first valve 11. The first valve 11 is able to be opened or closed by hand or by remote control. The first valve 11 either allows or does not allow the circulation of liquid-form gas 31 within the main pipe 6.

(15) The flexible second portion 8 is positioned in continuity with the first portion 7. The flexible second portion 8 is connected to the first portion 7 via a first end 81 of the flexible second portion 8. As a result, the flexible second portion 8 is secured to the articulated support device 26 on account of the connection between the first portion 7 and the first end 81 of the flexible second portion 8, and on account of the fact that the first portion 7 is supported by the bearer 30 of the articulated support device 26.

(16) The third portion 9 is present at the liquid-form gas receiving unit 3 and comprises a first termination 91 equipped with a header 15 and a second termination 92 which opens into the receiving tank 5. The header 15 allows the connection of the main pipe 6 so that the latter can connect the source tank 4 to the receiving tank 5 and thus allow the transfer operation to be performed. The second termination 92 of the third portion 9 is at least partially inserted into an internal volume of the receiving tank 5 so that the latter tank can receive the liquid-form gas 31 coming from the source tank 4 during the transfer operation. The third portion 9 comprises a second valve 12 positioned between the header 15 and the second termination 92 of the third portion 9. The second valve 12, just like the first valve 11, allows or prevents the transfer of liquid-form gas 31 from the source tank 4 to the receiving tank 5. Thus, to ensure that the transfer operation takes place, the first valve 11 and the second valve 12 need to both be opened so that the liquid-form gas 31 can circulate from the source tank 4 to the receiving tank 5.

(17) In order for the transfer operation to be able to be performed, a second end 82 of the flexible second portion 8 needs to be connected to the header 15 of the third portion 9. The flexible second portion 8 can thus be brought closer to the liquid-form gas receiving unit 3 by virtue of the articulated support device 26 and then the second end 82 of the flexible second portion 8 is then connected to the header 15 of the third portion 9. Following this operation, the main pipe 6 is completely connected, and the transfer 5 operation may begin. The flexibility of the flexible second portion 8 facilitates the connection between the flexible second portion 8 and the third portion 9. The first portion 7 and the third portion 9 may be flexible or rigid.

(18) Before the transfer system is set in place, the first portion 7 and the flexible second portion 8 are stored on the liquid-form gas source unit 2, just like the return line 18 is. The third portion 9 is stored on the liquid-form gas receiving unit 3.

(19) The return pipe 18 is connected upstream of the second end 82 of the flexible second portion 8 and therefore extends from the main pipe 6 until it opens into the source tank 4. The return pipe 18 comprises a first end 181 connected to the main pipe 6, and a second end 182 at least partially inserted into an internal volume of the source tank 4. The return pipe 18 allows the return of the liquid-form gas 31 to the source tank 4 when the transfer operation is complete and liquid-form gas 31 still remains in the main pipe 6. The details regarding the drainage operation will be set out hereinafter.

(20) The return pipe 18 comprises a first flow controller 19, a second flow controller 20, a third flow controller 21 and a fourth flow controller 22. The first flow controller 19 and the second flow controller 20 are situated at the first end 181 of the return pipe while the third flow controller 21 and the fourth flow controller 22 are situated at the second end 182. From a terminology standpoint, the term flow controller is differentiated from the term valve in so far as the flow controllers are situated only on the return pipe 18. The four flow controllers allow the liquid-form gas 31 to circulate in the return pipe 18 during the drainage operation. Incorporating a plurality of flow controllers into the return pipe 18 makes it possible to regulate the flow rate of the liquid-form gas 31 circulating in the return pipe 18, there being the risk that a flow rate that is too high and too sudden may damage the return pipe 18. During the transfer operation, all of the flow controllers are closed to prevent the liquid-form gas 31 from circulating in the return pipe 18.

(21) The return pipe 18 comprises an emergency disconnection device 23. The emergency disconnection device 23 allows the return pipe 18 to be separated from the main pipe 6 when too much mechanical stress is applied to the return pipe 18. Such mechanical stress may for example be caused by too much swell leading to tension in the return pipe 18.

(22) The return pipe 18 also comprises a pressure sensor 24 and a temperature sensor 25 which respectively measure the pressure and the temperature within the return pipe 18. Such measurements allow the drainage operation to run smoothly, as will be described in detail hereinafter.

(23) The return pipe 18 is connected to the main pipe 6 via a first connection/disconnection device 16. The flexible second portion 8 is connected to the header 15 of the third portion 9 by a second connection/disconnection device 17. Each of these connection/disconnection devices allow fluidtight and secure connection.

(24) The transfer system 1 also comprises a pressurizing line 10 connected to the first portion 7 of the main pipe 6. The pressurizing line 10 is able to input a fluid into the main pipe 6 and the return pipe 18. Such a fluid may, for example, be an inerting fluid such as molecular nitrogen, and may be used during the drainage operation to drive the liquid-form gas 31 in the main pipe 6 and in the return pipe 18. The pressurizing line 10 comprises a third valve 13 and a fourth valve 14. The fluid is able to come from the pressurizing line 10 if the third valve 13 and the fourth valve 14 are both open.

(25) The transfer system 1 may also comprise a gas line 36 which connects a headspace of the source tank 4 to the first portion 7 of the main pipe 6. A gas valve 37 is positioned on the gas line 36 and allows or does not allow a gas phase of the liquid-form gas 31 of the source tank 4 to circulate in the main pipe 6. Just like the pressurizing line 10, the gas line 36 may contribute to the operation of draining the liquid-form gas that has remained in the main pipe 6. The various variants of the drainage operation will be set out in detail hereinafter.

(26) Thus, during the transfer operation, and once the flexible second portion 8 has been connected to the header 15 of the third portion 9, the flow controllers of the return pipe 18 are closed and the first valve 11 and the second valve 12 are opened. The third valve 13 and the fourth valve 14 are also closed. The pump 32 situated on the main line 6 is set in operation and draws in the liquid-form gas 31 that is in the source tank 4. The liquid-form gas 31 therefore circulates in the first portion 7, the flexible second portion 8 and the third portion 9 until it flows out into the receiving tank 5. The transfer operation continues until the receiving tank 5 is full or until it reaches a filling corresponding to the demand from the liquid-form gas receiving unit 3. Once this has happened, the pump 32 is stopped and the drainage operation can then commence.

(27) FIG. 2 depicts the transfer system 1 according to the same embodiment as in FIG. 1. However, FIG. 2 illustrates a positioning of the transfer system 1 when the transfer operation is complete and the drainage operation has commenced. Because drainage is achieved under the effect of gravity, FIG. 2 illustrates a plurality of heights for certain elements of the transfer system 1. Each of these heights is defined as a function of a height reference H.sub.0 which may for example correspond to sea level. Three heights are thus illustrated. A first height H.sub.1 corresponds to a height of the second end 182 of the return pipe 18. A second height H.sub.2 corresponds to a height of the first end 181 of the return pipe 18 and to a height of the second end 82 of the flexible second portion 8 of the main pipe, these two ends in FIG. 2 being situated at the same height relative to one another. Finally, a third height H.sub.3 corresponds to a height of the first end 81 of the flexible second portion 8 of the main pipe 6.

(28) Once the transfer operation is complete, the second valve 12 is closed. The first valve 11 may also be closed depending on the variant of drainage method used. Thus, a part of the main pipe 6 situated upstream of the second valve 12, or between the first valve 11 and the second valve 12, is isolated from the receiving tank 5 so as to avoid potential returns of liquid-form gas in the main pipe 6. It is the liquid-form gas 31 that has remained in the main pipe 6 upstream of the second valve 12 or between the first valve 11 and the second valve 12 that will be drained off during the drainage operation.

(29) After that, the jib 29 of the articulated support device 26 is lifted upward. To do that, the pivot 28 performs a rotation 33 in the counterclockwise direction, thus causing the jib 29 to be raised. The objective of this movement of the articulated support device 26 is to increase the third height H.sub.3 relative to the height of the first end 81 of the flexible second portion 8, so that the third height H.sub.3 is greater than the second height H.sub.2, relative to the height of the second end 82 of the flexible second portion 8. The second end 82 of the flexible second portion 8 is thus vertically lower down than the first end 81 of the flexible second portion 8. The fact that the third height H.sub.3 is greater than the second height H.sub.2 means that the liquid-form gas 31 that has remained in the main pipe 6 can be made to circulate under the effect of gravity so that it collects at the second end 82 of the flexible second portion 8. By doing that, the formation of pockets of liquid-form gas 31 in the main pipe 6 is avoided, leading to an optimal drainage operation.

(30) The drainage operation continues by the first flow regulator 19 of the return pipe 18 being opened fully, and then with the progressive opening of the second flow regulator 20, the third flow regulator 21 and the fourth flow regulator 22 also being opened. By opening each of the flow regulators of the return pipe 18, the liquid-form gas 31 will flow therein. It is possible to initiate the flowing of the liquid-form gas 31 in the return pipe 18 by irrigating the main pipe 6, for example with a jet of seawater 34. A pressure differential is thus created and encourages the liquid-form gas 31 to flow in the return pipe 18.

(31) The liquid-form gas 31 is drained under the effect of gravity. In other words, the second height H.sub.2, relative to the height of the first end 181 of the return pipe 18, is greater than the first height H.sub.1, relative to the height of the second end 182 of the return pipe 18. The liquid-form gas 31 thus flows naturally in the return pipe 18 until it flows out into the source tank 4, the second end 182 of the return pipe 18 being vertically lower down than the first end 181 of the return pipe 18. Advantageously, the difference in height between the first height H.sub.1 and the second height H.sub.2 is at minimum of the order of three to four meters, so as to encourage the liquid-form gas 31 to flow.

(32) During the course of the drainage operation, the temperature sensor 25 measures the temperature within the return pipe 18. The temperature measurement makes it possible to check whether any liquid-form gas 31 remains in the return pipe 18. If the temperature is above a determined minimum temperature threshold, that means that there is no longer any liquid-form gas 31 remaining in the return pipe 18.

(33) If, despite the gravitational drainage, the temperature in the return pipe 18 does not increase, that means that some liquid-form gas 31 still remains in the return pipe 18. There are a number of possible ways of optimizing drainage.

(34) It is, for example, possible to close the first valve 11 if this is open, and then open the third valve 13 and the fourth valve 14 of the pressurizing line 10. The latter will then allow fluid to circulate to drive the liquid-form gas 31 that has remained in the main line and/or in the return line 18 and that has not been removed under gravity. The fluid in the pressurizing line 10 may for example be an inert gas such as molecular nitrogen. The opening of the third valve 13 and of the fourth valve 14 is possible only if the pressure in the return pipe 18 is sufficiently low, for example below 3.Math.5 bar. The pressure in the return pipe is checked using the pressure sensor 24. It is thus the pressure sensor 24 that determines the moment at which the fluid in the pressurizing line 10 can be input into the main pipe 6 and into the return pipe 18. Rather than driving the liquid-form gas 31, the fluid from the pressurizing line 10 may also circulate at ambient temperature so as to cause the liquid-form gas 31 that has remained in the main pipe 6 to evaporate.

(35) Alternatively, it is possible to optimize drainage by opening the gas valve 37, thus allowing a vapor phase of the liquid-form gas 31 to circulate in the gas line 36 and then in the main pipe 6. It is therefore this vapor phase of the liquid-form gas 31 that will drive the liquid-form gas 31 that has remained in the main pipe 6. Obviously, for such an alternative to work, the first valve 11 needs to be opened.

(36) When the temperature measured by the temperature sensor 25 is estimated to be high enough for it to be possible to consider that there is no longer any liquid-form gas 31 in the return pipe 18, then the drainage operation is complete. The flexible second portion 8 of the main pipe 6 can then be disconnected from the header 15 of the third portion 9, and the return pipe 18 can be disconnected from the main pipe 6. The liquid-form gas receiving unit 3 is thus filled and disconnected and may thus for example perform its mission of transporting the liquid-form gas 31 that has just been loaded into the receiving tank 5 or may consume the liquid-form gas 31 for propulsion.

(37) FIGS. 3 and 4 depict a second embodiment of the transfer system 1 according to the invention. For this second embodiment, only a position at which the return pipe 18 is connected to the main pipe 6 differs from the first embodiment depicted in FIGS. 1 and 2. Thus, only this connection will be discussed in the description specific to the second embodiment, and reference will be made to FIGS. 1 and 2 for the description of the parts that the two embodiments have in common.

(38) In FIGS. 3 and 4, the connection of the return pipe 18 to the main pipe 6 is made at the third portion 9. More specifically, the first end 181 of the return pipe 18 is situated downstream of the header 15 and upstream of the second valve 12. Such a positioning that differs from the first embodiment alters neither the operation of transferring the liquid-form gas 31 nor the operation of draining the liquid-form gas 31. The first end 181 of the return pipe does, on the other hand, of necessity need to be positioned upstream of the second valve 12 so that the latter valve can isolate the receiving tank 5 from the main pipe 6 by being re-closed.

(39) FIG. 4 is the counterpart of FIG. 2, for the second embodiment. In other words, FIG. 4 depicts the second embodiment of the transfer system 1 during the drainage operation. The drainage method is identical to that of the first embodiment. Thus, the second valve 12 and possibly the first valve 11 are closed, then the articulated support device 26 raises the jib 29, thus creating the height differential between the second height H.sub.2 and the third height H.sub.3. The second height H.sub.2 is itself greater than the first height H.sub.1 to ensure drainage under the effect of gravity. The liquid-form gas 31 can thus flow into the return pipe 18 after the flow controllers thereof have been opened.

(40) FIG. 5 is a flowchart depicting the drainage method 100 for draining the liquid-form gas according to the invention. The drainage method 100 is initiated once the transfer operation is complete. The end of the transfer operation is marked by the stopping of the pump used for circulating the liquid-form gas from the source tank to the receiving tank. The drainage method 100 begins with a first step 101 during which the second valve of the third portion is also closed. Closing the second valve allows the receiving tank to be isolated when the liquid-form gas that has remained in the main pipe is circulating in the return pipe. Specifically, there is a risk of liquid-form gas returning from the receiving tank to the return pipe, for example as a result of a pressure differential between the receiving tank and the return pipe. Such a situation is liable to arise in particular if the liquid-form gas transfer system is installed in accordance with the second embodiment as the first end of the return pipe is situated at the level of the third portion, and therefore closer to the receiving tank than in the first embodiment. The closing of the second valve alleviates this disadvantage, which is why the second valve is systematically downstream of the connection of the return pipe whatever the embodiment of the transfer system. It will be appreciated that the first step 101 absolutely must be performed before any other step of the drainage method 100 in order to ensure its smooth operation. Thus, the liquid-form gas that has remained in the main pipe part situated upstream of the second valve is the gas with which the drainage method 100 is concerned.

(41) Once the first step 101 has been completed, the drainage method 100 continues with a second step 102 in which the articulated support device is raised so as to create the height differential between the first end of the flexible second portion and the second end of the flexible second portion. The latter is in fact liable to create pockets of liquid-form gas as a result of its flexibility. The potential creation of such pockets may lead to difficulties in circulating the liquid-form gas as far as the return pipe. Raising the articulated support device thus makes it possible to create an appreciable height differential between the two ends of the flexible second portion and thus collect the liquid-form gas that has remained in the main pipe in its entirety or almost its entirety. The liquid-form gas that has remained in the main pipe is thus amassed near the first end of the return pipe so that it can be more easily and effectively drained into the return pipe. In order to bring about optimal flow of the liquid-form gas in the return pipe, the first step 101 and the second step 102 need both to be performed before the liquid-form gas is allowed any access whatsoever to the return pipe.

(42) It is not until a third step 103 that all of the flow controllers on the return pipe are opened. To avoid damaging the return pipe by causing too strong or too sudden a flow of liquid-form gas therein, at least one of the flow controllers may be opened progressively.

(43) Once the third step 103 has been performed, the drainage method 100 may end directly with an end step 106. However, the drainage method 100 may comprise a fourth step 104 and/or an additional step 105 allowing the drainage operation to be optimized.

(44) The fourth step 104 makes it possible to initiate a flow of liquid-form gas in the return pipe in the event of a blockage caused by the pressure in the main pipe and/or the return pipe. Thus, the fourth step 104 consists in heating the main pipe in order to cause the liquid-form gas that has remained in the main pipe to start to evaporate and thus create a pressure differential encouraging the liquid-form gas to flow in the return pipe. The heating of the main pipe may for example be achieved using a jet of seawater, as depicted in FIGS. 2 and 4, but any other means of causing the temperature of the liquid-form gas to rise and that is suited to the context of the invention, is conceivable.

(45) The additional step 105 may be performed after the first step 101 as depicted in FIG. 5, but the additional step 105 can be performed at any moment in the sequence of the drainage method 100 provided that moment comes later than the first step 101. The additional step 105 may proceed according to a number of variants.

(46) A first variant is to close the first valve so as to isolate a section of the main pipe containing liquid-form gas. The fourth step 104 may then suffice to cause the liquid-form gas that has remained in the tank to drain off under the effect of gravity, and the drainage method 100 may thus end.

(47) If this is not enough, it is possible to use a second variant of the drainage method 100. The second variant may be used after the use of the first variant, or else may be used immediately without proceeding via the first variant. The second variant consists first of all in closing the first valve, then injecting the fluid from the pressurizing line, doing this by opening the third valve and the fourth valve, into the main pipe and the return pipe. The purpose of this second variant is to drive the liquid-form gas that has remained in the main pipe or the return pipe and that has not been removed under the effect of gravity. The fluid thus drives the liquid-form gas toward the source tank through the main pipe and the return pipe. The second variant of the additional step 105 thus makes it possible to complete the drainage operation in a way that is reliable, so that it is possible to be certain that there is no longer any liquid-form gas remaining in the main pipe and in the return pipe. Execution of the second variant is dependent on the pressure in the return pipe. The pressure needs to be low enough, for example below 3.Math.5 bar, and it is the pressure sensor present on the return pipe that is used to check whether the second variant can proceed. In this second variant, the fluid used is different than the liquid-form gas and may for example be molecular nitrogen.

(48) It is also possible to use a third variant of the additional step 105. The third variant differs from the first variant and from the second variant notably in that the first valve needs to remain open so that the third variant can be applied. The third variant consists in opening the gas valve, so as to connect the headspace of the source tank to the main pipe via the gas line. It is thus the vapor phase of the liquid-form gas that will circulate in the main pipe and drive the liquid-form gas remaining in the main pipe as far as the return pipe.

(49) In order to complete the drainage method 100 using the end step 106, it is advantageous to check that no liquid-form gas remains in the return pipe. This check is at least partially provided by the temperature sensor present on the return pipe. A sufficiently high temperature, for example a temperature higher than ?85? C., in the return pipe confirms that there is no longer any liquid-form gas in the return pipe. The drainage method 100 may then end with the end step 106, which is a sign that the main pipe and the return pipe can be disconnected in complete safety.

(50) FIG. 6 depicts an example of a maritime terminal provided with the transfer system 1, the latter comprising the main pipe 6 and the return pipe 18. The transfer system 1 allows the liquid-form gas to be transferred from the liquid-form gas source unit 2, which in this instance is a fixed offshore installation. The transfer system 1 allows the liquid-form gas receiving unit 3, which in FIG. 6 is illustrated as being a transport ship 35 and comprises the receiving tank 5 to be loaded, the loading being performed from the liquid-form gas source unit 2. This unit comprises at least the source tank 4 connected to the transfer system 1.

(51) Once the liquid-form gas has been transferred from the source tank 4 to the receiving tank 5, and in order to disconnect the main pipe 6 in complete safety, the drainage method described in FIG. 5 is implemented so that the liquid-form gas that has remained in the main pipe 6 returns to the source tank 4 by passing along the return pipe 18.

(52) Of course, the invention is not restricted to the examples that have just been described and numerous adaptations may be made to these examples without departing from the scope of the invention.

(53) The invention, as has just been described, does indeed achieve its stated objective and is able to propose a liquid-form gas transfer system comprising a main pipe and a return pipe allowing the liquid-form gas to return to its starting point using drainage under the effect of gravity. Variants not described here may be implemented without departing from the context of the invention, provided that they comprise a liquid-form gas transfer system in accordance with the invention.